A technique for performing a high-quality optical transmission in an optical transmission system, which realizes a high-speed transmission with use of optical fibers, is disclosed by Document 1.
According to this document, the optical transmission system includes an optical transmitter and an optical receiver, which are connected to each other by two optical fibers.
The optical transmitter intensity-modulates an optical signal based on a radio frequency signal (hereinafter called the “RF signal”) that requires to be transmitted. Then, the optical transmitter generates two optical signals in antiphase from the intensity-modulated optical signal, and separately transmits the generated two optical signals to the optical receiver via the respective optical fibers.
The optical receiver receives the two optical signals from the optical transmitter via the respective optical fibers, and converts the received two optical signals into electrical signals respectively. Then, the optical receiver changes one of the electrical signals to be in antiphase, and adds the changed electrical signal in antiphase to the other electrical signal.
According to this operation, the RF signals in antiphase, which are the intensity-modulated components of the two optical signals received from the optical transmitter, become in in phase after converted into electrical signals, and these electrical signals in in phase are to be added to each other. On the contrary, two optical-signal noise components in in phase received from the optical transmitter are to be in antiphase at the time of the addition. Therefore, the noise components cancel out each other. This realizes a high-quality optical transmission.
However, with the above-described optical transmission system, two optical fibers are required between the optical transmitter and the optical receiver for canceling out the noise components. This is a cause of the high cost for constructing an optical transmission system. Therefore, it is difficult to apply the conventional optical transmission system to systems for commercial use.
Document 1
“IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES”, published by IEEE, VOL. 46, NO. 12, PP. 2083-2091, DECEMBER 1998.